Fix gnu builddir

I think I did finally find a way to make this work, by returning to my old (and I completely forgot the context in which it had first arisen originally...) idea about `ANCHOR_TOP_BUILDDIR`. I (vaguely) remember that you didn't like it, but I think it's unavoidable to allow distinguishing between the two cases here and everything seems to work with it, so I'd really, really like to add it.